A thin film transistor (TFT) is a particular type of field effect transistor, made by depositing thin films of a semiconductor active layer, a dielectric layer, and metallic contacts over a supporting substrate. The primary application of TFTs is in liquid crystal displays, and for this reason the most common substrate is glass. This differs from conventional transistors used in electronics, where the semiconductor material, typically a silicon wafer, is the substrate. Transparent TFTs (TTFTs) are particularly desirable for displays that rely on pixel-by-pixel modulation of light emitted by a backlight.
TFTs can be made using a wide variety of semiconductor materials. A common material is silicon. The characteristics of a silicon-based TFT depend on the silicon crystalline state. The semiconductor layer can be amorphous silicon or microcrystalline silicon, or it can be annealed into polysilicon. Other materials which have been used as semiconductors in TFTs include compound semiconductors such as cadmium selenium (CdSe) and metal oxides such as zinc oxide. TFTs have also been made using organic materials (Organic TFTs or OTFTs).
The glass substrates used in typical liquid crystal displays cannot withstand the high temperatures characteristic of polysilicon transistor fabrication. For this reason, amorphous silicon, because of its low dark conductivity and relatively easy fabrication on large area substrates at moderate temperatures, is a very effective active layer material for high resolution large area displays. The most common TFTs in use today are based on hydrogenated amorphous silicon (“a-Si:H”) as the semiconductor active layer.
Chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PE-CVD), and physical vapor deposition methods such as sputtering are most commonly employed for the deposition of the silicon, insulating, and conducting layers that constitute a TFT. Solution-processed transparent TFTs based on chemical precipitation of zinc oxide and silicon dioxide have also been reported.
In a transistor, there are typically three electrodes, serving as gate, source, and drain. The gate electrode supplies the controlling voltage to the transistor, and the semiconductor channel of the transistor conducts current from the source to the drain in response to the gate voltage. The gate insulator, or gate dielectric, of a TFT electrically insulates the gate from the semiconductor channel. Superior performance in a TFT requires a high channel conductance, fast “on” and “off” responses to the applied gate voltage, a very rapid increase and decrease in the source-to-drain current as the gate voltage rises and falls past a threshold switching value, minimal current leakage from the source to the drain in the absence of an applied gate voltage and negligible current leakage from the channel to the gate. These operating characteristics should be stable, and should not change or drift after a long period of applied gate voltage.
The gate dielectric material plays a critical role in determining the performance of a TFT. In general, a thinner gate dielectric layer leads to a greater voltage gradient across the gate dielectric layer, and this in turn leads to the more rapid generation of more charge carriers in the semiconductor, and permits a reduction in driving voltage. The properties of the gate dielectric material set a limit on how thin this layer can be. The material must not break down and conduct current under the influence of the voltage gradient, it must bind to the semiconductor channel material without leaving too many dangling bonds (“interface states”), it must be tough enough to withstand thermal cycling without fracturing or separating from the various materials it is bonded to (typically, the substrate, gate, and dielectric layers), and it must exhibit stable properties under extended application of a gating voltage. Furthermore, in order to be commercially feasible, the material should be easily laid down and patterned using existing microfabrication technology. It should be capable of producing extremely thin layers with extraordinary uniformity, because large displays can contain millions of pixels, and the acceptable defect rate among the millions of TFTs in each such display is generally zero. Among the most suitable materials for TFTs are silicon oxide (SiOx) and silicon nitride (SiNx). Although silicon nitride is generally considered to be the superior gate dielectric for a-Si:H TFTs, there are disadvantages to SiNx as an gate dielectric: it is brittle, subject to thermal stress upon cooling, and has relatively low transparency. Prolonged application of a gating bias can cause charge trapping at the interface with the a-Si:H, which leads to a shift in the threshold voltage of the TFT. There remains a need for more stable, flexible and transparent gate dielectric materials, especially for use on flexible substrates.
In addition to the interface with the gate dielectric, the interface on the other side of the a-Si:H channel, the so-called “back-channel”, also influences the performance of the transistor. In particular, the etching and subsequent application of a dielectric “passivation layer” to the a-Si:H back channel can lower the density of surface states, and control surface leakage and photoleakage currents. In a back channel etched (BCE) a-Si:H TFT, a passivation layer is necessary to protect the back channel from damage and contamination during subsequent processing. For conventional BCE TFT devices, PECVD-deposited silicon nitride (SiNx) is commonly used as a passivation layer. However, SiNx, due to its relatively high dielectric constant, high stress and low transmittance, constrains the size and performance of the device. There remains a need for lower-dielectric, lower-stress, and higher-transmittance dielectric materials for back-channel passivation in TFTs.
Liquid crystal display panels are increasingly popular for computer display screens and flat-panel television sets. The market for these commercial products continuously demands larger-sized displays, higher resolutions, and higher color image rendering capabilities. There is a need for thin film transistors, suitable for use as switching devices in active-matrix displays, that are economical to manufacture, with low defect rates and improved electrical characteristics, such as high field effect mobility, reliability against high frequency, and low leakage current.
Organic high-emitting displays are a new technology for flat-panel displays. Commercial production of OLED displays is accelerating rapidly, because of their several advantages over liquid-crystal displays. Organic light-emitting displays rely on thin-film transistors that continuously provide direct current, and as a result there is a need for transistors that are particularly stable in long-term use.
Volatile silanes and silicones are commonly used as silicon source gases for plasma enhanced chemical vapor deposition (PE-CVD) growth of carbon-containing silicon oxide films, sometimes referred to as organosilicate glass (OSG). OSG is commonly used for insulating layers (low-k dielectrics) between passive conductive elements, and as the underlying insulator in damascene processes. At high oxygen/silicone ratios, the resulting films are harder and more silica-like than at low oxygen/silicone ratios, and such films have been used as hard protective coatings on polymers and metals.